A conventional ARQ (Automatic Repeat reQuest) system has three well known systems of “Stop and Wait ARQ (SW-ARQ)”, “Go back N ARQ (GBN-ARQ)” and “Selective Repeat ARQ (SR-ARQ). These three systems have merits that a simple decoder can be employed therein in comparison with an error correction system and high reliability can be obtained and so forth.
In the first place, operation of “SW-ARQW” system is described while employing FIG. 1. A base station transmits one cell (or one packet) to a mobile station. The mobile station checks whether or not an error occurs about the cell in a transmission network. In the case of no error, the mobile station transmits an acknowledge signal (ACK) to the base station while employing a returning channel to inform that the mobile station has received data correctly. While in the case where the mobile station has detected the error, the mobile station transmits a retransmission request signal (NAK; Negative AcKnowledge) of the cell to the base station while canceling the cell with the error. When the base station receives the NAK, the base station retransmits data that is stored in a transmission buffer to the mobile station. Further, the retransmission is continued until the ACK is returned from the mobile station. Thus, since procedure of the “SW-ARQ” system is simple, the “SW-ARQ” system is employed widely for many data transmission.
Further, in the “GBN-ARQ” system, the base station transmits the cell continuously. Then, the base station, as illustrated in FIG. 2, transmits a next cell without waiting a response signal from the mobile station about the cell. Consequently, the base station results in transmitted plural cells until the base station receives the response signal from the mobile station. In an example of FIG. 2, the base station transmits the cells of NO. 1 to NO. 5 until the base station receives the NAK from the mobile station. Further, when the base station receives the NAK from the mobile station, the base station retransmits from a cell (NO. 2 cell) with an error to a cell (NO. 6 cell) which is transmitted in a timing when the NAK is received to the mobile station while going back to a cell corresponding to the NAK stored in the transmitting buffer. The mobile station cancels the cells from NO. 2 to NO. 6 received previously, because the cells NO. 2 to NO. 6 are retransmitted from the base station.
Moreover, concerning the SR-ARQ system, likewise the GBN-ARQ system, the base station transmits the cell continuously. However, in this system, as illustrated in FIG. 3, the base station retransmits only the cell with occurrence of an error to the mobile station. The SR-ARQ system is one whose transmission efficiency is the most excellent among the above-described three ARQ systems.
Here, in an asymmetrical data transmission, there is a problem that load of a forward channel from the base station to the mobile station becomes large in comparison with a load of a reverse channel from the mobile station to the base station.
In the above-described conventional ARQ system, since retransmission of information data with occurrence of an error in reverse/forward channel is performed while employing the same reverse/forward channel, a problem that load of the forward channel is large in comparison with a load of the reverse channel is not solved.